Plasma Arc Welding constricts the arc through a tiny orifice, superheating it into a focused plasma column. The result: deep single-pass penetration, extreme stability, and the smoothest automated welds in industry.
In TIG the arc is a free cone. PAW pushes that same arc through a copper nozzle with a small orifice, constricting it into a narrow column of ionized gas — plasma — far hotter and more directional. A pilot arc starts inside the torch, then the main arc transfers to the work.
Two modes matter. Melt-in mode behaves like a stable TIG arc for thin and precision work. Keyhole mode at higher current bores completely through the joint — the molten metal flows around the hole and freezes behind the arc for a full-penetration weld in one pass. That is the trick that makes PAW the choice for automated tube and aerospace welding.
Orifice size and current decide melt-in vs keyhole.
| Material | Orifice | Plasma gas | Amps | Mode |
|---|---|---|---|---|
| 0.030 in steel | 0.9 mm | Argon | 20–40 | Melt-in |
| 1/16 in steel | 1.2 mm | Argon | 60–100 | Melt-in |
| 1/8 in steel | 1.6 mm | Argon | 120–170 | Keyhole |
| 1/4 in steel | 2.0 mm | Ar/H2 | 180–250 | Keyhole |
| 1/8 in stainless | 1.6 mm | Ar/5% H2 | 130–180 | Keyhole |
| 1/8 in aluminum | 2.0 mm | Argon | 160–210 | Keyhole |
| Property | TIG | PAW |
|---|---|---|
| Arc shape | Free cone | Constricted column |
| Energy density | Low | High |
| Arc length tolerance | Tight | Very forgiving |
| Full penetration | Many passes | One keyhole pass |
| Automation ease | Good | Excellent |
| Nozzle wear | None | Orifice wears |
CC source with pilot arc, upslope/downslope, and gas sequencing — the control brain of the cell.
Browse power sources →Water-cooled torch with consumable orifice nozzles, usually torch-mounted on a fixture.
Browse torches →Dual flowmeters and solenoid sequencing for plasma and shielding gas — timing is everything.
Orifice nozzles, tungsten, and insulators — a wear set, replaced on a schedule.
Browse consumables →Carriages, positioners, and orbital heads — PAW shines when the torch moves mechanically.
Browse automation →Argon for plasma; argon, helium, or hydrogen mixes for shielding. Two independent supplies.
Water cooling for torch and consumables at high current — never run hot without it.
Melt-in mode welds every position. Keyhole mode wants the joint flat or gently rotated.
Both modes shine here — the natural home of keyhole full-penetration work.
Melt-in handles horizontals; keyhole needs a slight torch tilt to hold the hole.
Melt-in only, low current — plasma arc is very stable out of position.
Orbital heads rotate the torch around fixed pipe — keyhole quality in the 5G position.
PAW rewards setup discipline more than torch skill — the arc holds itself steady if you feed it right.
TIG-like dabbing and stringers at low current — precision work on thin gauge.
Hold at the joint start until the hole forms — you see the root open, then travel.
Ramp current down at the end so the hole fills and seals — no exit crater.
Pulsed current controls heat on thin and out-of-position work — the arc never flickers.
The torch rides a head around fixed pipe — the classic automated PAW application.
Keyhole root plus melt-in fills on thick plate — combine both modes on one joint.
PAW defects are mostly keyhole physics — heat, travel, and gas in balance.
Appearance
Molten metal falls back into the hole — the weld fills uneven or drops through.
Causes
Too much current, too slow travel, or a gap too wide.
Prevention
Drop amps, speed up, and tighten fit-up.
Appearance
The puddle erupts — holes and ragged edges on both sides of the joint.
Causes
Plasma gas flow too high or travel too slow for the current.
Prevention
Trim plasma flow and balance speed with amps.
Appearance
Root never fused — the bead sits on the joint with a visible cold line.
Causes
Amps below the keyhole threshold or travel too fast.
Prevention
Raise current, slow down, confirm the hole actually formed.
Appearance
Pin holes in the bead or root — the classic contamination signature.
Causes
Dirty metal, no back purge, or moisture in the gas.
Prevention
Clean, purge, and dry the gas lines.
Appearance
Grooves along the bead toes — the arc pressure carved the sidewalls.
Causes
Too much current or an off-center arc.
Prevention
Center the torch and trim the current.
Appearance
Bright flecks in the weld and a wobbling arc — tungsten touched the puddle.
Causes
Contact with the work or spatter bridging the orifice.
Prevention
Keep standoff, inspect the nozzle, re-dress the tungsten.
Appearance
The arc wanders or flickers — the bead ripples unevenly.
Causes
Worn orifice, bad tungsten prep, or plasma flow issues.
Prevention
Fresh consumables, sharp tungsten, steady gas.
Appearance
Star crack at the weld end — current stopped before the crater filled.
Causes
Abrupt arc termination without downslope.
Prevention
Programmed downslope and post-fill in the sequence.
The documents behind PAW electrodes, gases, and qualification.
| Standard | Covers |
|---|---|
| AWS A5.12 | Tungsten electrode classification and color coding |
| AWS A5.18 / A5.28 | Solid filler wire for carbon and low-alloy steel |
| ISO 14175 | Shielding and plasma gas classification |
| AWS D1.1 / D1.6 | Structural steel and stainless welding codes |
| ASME Section IX | Procedure and welder qualification for pressure equipment |
| AWS B2.1 | Procedure qualification of welding processes |
| AWS C5.1 | Recommended practices for plasma arc welding |
| ISO 2553 | Symbols for welded joints on drawings |
All-in-one machine settings helper for a fast PAW starting point.
Set plasma and shielding flows and read the gas usage you need.
Match filler to base metal for melt-in and keyhole work.
Volts × amps ÷ travel speed — the number that controls distortion and cracking.
What orifice and current fit foil vs 3/8 in plate.
See keyhole establishment and melt-in technique on real metal.
Watch the technique sections — keyhole, melt-in, and orbital PAW — demonstrated on real metal.
Watch VideosA shop-floor cheat sheet with the parameter table, orifice chart, and gas quick reference.
Download from Library